Rear Brake Problem

Hi ,can any one shed some light on this problem.brakes on my 94 spi cooper were juddering seemed to be coming from the back because when you pull the h/brake up slowly you can feel the judder quite badly,not long replaced the fronts .so i fitted a set of alloy minifins (replicas}but then the judder was even worse.so pulled them off and bought a normal set of rear drums fitted these and it was better but still not completly gone.ok at slower speeds but when going 60/70 then braking up to a roundabout/junction they judder whats going on ?could it be the rear hubs are out ,cant be the shoes ,bit stumped,can anyone shed some light or had the same problem.cheers steve